Our application's objective is to understand how the brain processes learned communication sounds. Auditory neurons in the avian song control nucleus HVc are highly selective for the bird's own song. This proposal seeks to discover the origins of song-selective auditory responses exhibited by HVc neurons. Three features of song-selectivity motivate the search for its neural substrate. First, song-selectivity arises through auditory experience. Therefore, finding where these responses first emerge within the songbird's brain will guide us to the site where auditory experience alters neural function. Second, song-selectivity affords a powerful system to examine the biophysics of combination sensitivity, a process relevant to the auditory processing of human speech. Third, song-selectivity is likely to facilitate song perception. In short, discovering the neural mechanisms of song selectivity can illuminate how auditory experience shapes behaviorally-relevant neural sensitivity to learned communication sounds. The specific aims seek 1) to discover the nature of local contributions to song-selectivity in HVc; 2) to determine whether extrinsic sources provide song- selective information to HVc; and 3) to locate these extrinsic sources, if they exist. In vivo intracellular recordings will be used to record from identified HVc neurons to reveal the pattern of subthreshold inputs underlying song-selective responses. Whether presynaptic neurons providing song-selective subthreshold input onto HVc neurons are local or extrinsic to HVc will be explored by reversibly inactivating the HVc local circuit, then measuring whether song-selective subthreshold responses persist. To localize extrinsic sources of song-selective input to HVc, we will use spike-triggered averaging techniques in vivo, and we will use focal glutamate stimulation and intracellular staining in vitro to better understand the source and function of monosynaptic inputs onto identified HVc neurons. In short, we will combine in vitro and in vivo techniques to address the origins of song-selective responses in HVc, with the aim of understanding learned communication.